Tuned coupling system



Patented Oct. 6, 1936 UNITED STATES.

TUNED COUPLING SYSTEM Nelson P. Case, Great Neck, N. Y., assgnor'to Hazeltine Corporation, a corporation of Dela- Ware Application 4November 25, 1935, Serial No. 51,4120

zo claims. wieso- 20) f This invention relates to tuned coupling systems, and, while it is of general application, it is particularly suitable for use Vin tunable multiband radio broadcast receivers.

Double-tuned radio-frequency coupling systems are frequently employed in lieu of single-tuned coupling systems in order to' utilize the greater selectivity of the double-tuned systems, although their use islfrequently attended with problems more difficult of solution than those encountered with single-tuned coupling systems. A characteristic of double-tuned coupling systems is a fairly substantial variation in their gain or transfer ratio for different frequencies within a given band. 'I'his characteristic is generally so pronounced that it should not be neglected,l particularly inasmuch as it is generally desirable that the gain or transfer ratio of a coupling system be fairly uniform for different frequencies within a given band.

The foregoing characteristic of double-tuned coupling systems gives rise to the problem of procuring a desired slope or variation of the gain or transfer characteristic thereof with frequency. The solution of this problem is complicated by the desirability of maintaining the syntony of the two circuits of a double-tuned coupling system. This complication. renders unsatisfactory arrangements which, upon the adjustment of the coupling system to vary the slope of the gain or transfer characteristic, disturb the syntony of the two circuits.

The solution of the above problem is further complicated by practical considerations arising when the coupling system is used for frequencies (such as those above the general broadcast band) for which relatively small inductances are generally provided. Thus in such cases it is difficult to provide and controllnductive coupling effects of the relatively small magnitudes involved. When a double-tuned coupling system is variably tuned, itis desirable to tune both the rst and second tunable circuits thereof conjointly with each other and with other tunable circuits, by variable condensers having the same minimum and maximum values and the same law of capacitance variation. This renders unsatisfactory arrangements which, upon adjustment of the coupling system to vary the slope of the gain or transfer characteristic, produce misalignment of the first and the second tunable circuits with respect to each other or of either or both of such circuits with respect to any of the other tunable circuits tuned conjointly therewith.

With double-tuned multi-band ycoupling systems, such as are employedrin multi-band radio broadcast receivers, there is the additional problem of providing an arrangement having a gain or transfer ratio which is readily subject to control 5 from band to band. `This is desirable in order to compensate for the progressive decrease in the attainableA antenna-to-detector over-all gain of multi-band receivers as the bands in which the receiver is operated are higher in the frequency spectrum, so as to obtain an over-al1 gain which is more nearly uniform from band to band.

It is an object of this invention to provide a double-tuned coupling system in which the 4gain or transfer characteristic maybe given any dei5 sired slope without affecting the syntony or alignment of the first and second tuned circuits of the coupling system with respect to each other and/or without substantially affecting the syntony or alignment of either or both of such circuits with respect to related tuned circuits, and in which all the components thereof are such that the design of the system is commercially practical at any portion of the radio-frequency spectrum. f Q5 It is another object of the invention to pro-- vide a double-tuned coupling system for multiband radio receivers whichis effective automatically to control the over-all gain in accordance with the band in which the receiver is operated, such coupling system being of simple and economical construction and effective Without impairment of the selectivity or delity of the receiver.

A further object of the invention is the pro- 35 vision of a double-tuned coupling system in which the inductance elements may be constructed and mounted vin the receiver with ease and economy.

Other objects of this invention will become apparent from the specication taken in conjunction with the accompanying drawing and appended claims.

The invention contemplates, for a given band of frequencies, a coupling system comprising two tuned circuits having substantially similar reactance characteristics and at least one transfer means between the two circuits' having a transfer characteristic variable with frequency and consisting of a self-reactance network all the reactances of which are of the same type and effectively xed in value, the network having two similar aims of equal effective reactive value individually and symmetrically included in circuit dit with the tuned circuits, and having another arm symmetrically interconnecting the two similar 55 arms, the control of the transfer action of the transfer means being eected by altering the effective reactive value of the interconnecting arm. Generally, there are two transfer means between the two circuits, the two transfer means having transfer characteristics which vary in opposite senses with variations in frequency, so that the resultant transfer characteristic of the two transfer means is substantially uniform over a given range of frequencies, or at least is a closer approximation thereto than is obtainable with either transfer means alone.A In the preferred arrangement the two transfer means are capacitive in character, the first coupling portions of the two circuits of relatively low alternating potential and the second coupling portions of the two .circuits of relatively high alternating potential, with no inductive coupling existing between the two circuits. The inductance elements of the two circuits may thus be mounted in separate shields. This is of practical advantage in frequency bands in which inductive relationships are difficult to control and thus where individual shields for the several inductance elements are of great practical value.

Theyinvention contemplates, for two or more bands of frequencies, a coupling system for each band similar to that described for one band, with `the provision that the capacitance of each trans- Fig. 3 is a circuit diagram of a modified form of the coupling system of the receiver of Fig. 2.

Referring to Fig. 1, the superheterodyne receiver there shown comprises an antenna. circuit III connected to a radio-frequency selector and amplifier II, which may be ofv conventional type, including a variable tuning condenser I2. 'I'he output of selector and `amplifier Il is connected to the input of a variably'tuned doubletuned coupling systemgenerally designated I3, the output of which is connected to the input of an oscillator-modulator I4, which may be of conventional type, including a variable tuning condenser I5. In cascade with oscillator-modulator I4 are an intermediate-frequency amplifier I6, a detector and automatic amplification control source I1, an audio-,frequency amplifier I8 and a sound reproducer 20. The automatic gain con-l trol bias potential may be applied to one or more rof the radio-frequency selector and amplifier' II, oscillator-modulatorr I4 and the intermediate- 1 frequency amplier I6, as desired. The variable tuning condensers I2 and I5 are preferably ganged for unicontrol, as indicated'by the dotted lines 2l. Also ganged with these condensers are tho variable tuning condensers 3l and 35, as will be described hereinafter.

Considering first the operation of the receiver as a whole, and neglecting 4for a moment the par.- ticular characteristics of the coupling system I3, per se, the desired received signal wave is selected and amplified in the radio-frequency selector and amplifier II, further selected and transferred by the coupling system I3 to the osclllator-modula tor I4 where the signal is converted into a modulated intermediate frequency. The signal, as thus converted, is amplified in the intermediatefrequency amplifier I6, rectifled by detector I1,

I ment 30.

Aject to automatic control by the AVC bias potentials, according to the manner well understood in the art.

The coupling system generally designated I3 comprises two parallel tunable circuits 24 and 25 and, between such tunable circuits, two transfer means 26 and 21. The first tunable circuit 24 in cludes an inductance element 30 and a variable tuning condenser 3l, While the second tunable circuit includes an inductance element 34 and a variable tuning condenser 35. Inductance element 34 has the same value as inductance ele- Variable condensers 3| and 35, and preferably I2 and I5 also, have the same minimum and maximum values, and the same laws of capacitance variation. Tunable circuits 24 and 25 thus have similar reactance characteristics", which term is used herein to refer to two or more tuned circuits that are electrically similar from the viewpoint of their resonating properties so that normally, in the case of untunable circuits, they are syntonous with each other and, in the case of tunable circuits having conjointly actuated tuning elements, they are in alignment with each other over their respective tuning ranges.

The first transfer means 26 includes two seriesaligning condensers 32 and 36, of equal value, disposed in relatively low alternating potential portions of the tunable `circuits 24 and 25, respectively, and having a common terminal forming a neutral pointl suitable for grounding, as illustrated. This transfer means includes also an adf justably fixed condenser 38 interconnecting the junction of inductance element 30 and condenser 32 with the junction of inductance element 34 and condenser 36. The condensers 32 and 36 are preferably of relatively large value, at least three or four times the maximum value of the tuning condensers, while the condenser 38 has a value substantially less than that of condensers 32 or 36, and generally lying between the minimum and maximum values of the tuning condensers. 'I'he condenser 38 is thus effectively external to the tunable circuits 24 and 25 in the sense that the reactance thereof at the operating frequencies is very high relative to the reactance of condensers 32 and 36 so that, for normal adjustments of condenser 32, the resultant variations of reactance of the series combination of condensers 32 and 36, for example, have no appreciable effect on the tuning of the tunable circuit 25.

The second transfer means 2'I comprises a condenser connected between the high alternating potential terminals of inductance elements 30 and 34. Condenser 21 has a relatively small value, generally appreciably less than the minimum value of the tuning condensers, and may be adjustably fixed, as indicated. Inductance elements 30 and 34 are inductively unrelated, and thus are conveniently separately mounted, each within an individual grounded shield c an, as schematically illustrated. The two tunable circuits 24 and 25 are tuned to the frequency of the desired signal wave conjointly with each other and with the other. tunable circuits of the receiver.

It is to be observed that the twoy tunable circuits 24 and 25 are symmetrically coupled; that is, the arrangement of the two transfer means is such that each influences the resonant frequencies of both circuits in the same manner andto the same extent. The alignment of the two tunable circuits 24 and 25 is, therefore, not disturbed by adjustments of either of interconnecting condensers 21 or 38. Further, the alignment of tunable circuits 24 and 25 with respect to the other tunable circuits ofthe system is not disturbed to any substantial extent within the normal limits of the adjustment of condensers 38 and 21.

In considering the transfer action of the coupling system I3, the first transfer means 26 introduces into the second circuit 25 from the circuit 24, across series condenser 36, a voltage which is always a certain definite fraction of the voltage across series condenser 32.' The. voltage across series condenser 32 varies inversely with the frequency to which the circuits are tuned. The gain or transfer ratio of the first transfer 'means, therefore, varies inversely with' frequency. The fractional relationship of the Voltage across .condenser 36 to that across condenser 32, that is, the effectiveness of the rst transfer means, is subject to control by adjustment of interconnecting condenser 38.

The second transfer means 21 introduces, across the terminals of the second circuit 25, a voltage which varies directly with the frequency to which the first circuit is tuned, for the reactive impedance of interconnecting condenser 21 decreases, and the anti-resonant impedance of the f 38 alone.

second circuit 25 increases, with increases in the frequency to which the circuits are tuned. The gain or transfer ratio of the second transfer means 25 thus varies directly with frequency. The effectiveness of the second transfer means is subject to control by adjustment of interconnecting condenser 21.

The voltages introduced into the second circuit 25 by the transfer means 26 and 21 are substantially in phase, so that the transfer characteristic of the complete coupling system I3is substantially the arithmetic sum of their separate transfer characteristics.

The transfer characteristics of the two transfer means 26 and 2l thus vary in opposite senses, with the first transfer means 28 being principally effectiveat the low-frequency end of the band over which the coupling system is tunable, and with the second transfer means 21 being principally effective at the high-frequency end of such band. As a consequence the over-al1 transfer characteristic of the complete coupling system I3 may be justment of condensers 38 and 21 does not affectY the alignment of the ltwo circuits 24 and 25 with respect to each other, or, within the limits of such adjustments, to any substantial extentwith respect to the other tunable circuits of the system.

in certain cases, satisfactory control of the over-all transfer characteristic of coupling system I3 may be obtained by adjusting condenser Condenser 21 thus may be furnished as a fixed condenser, which is of commercial advantage since it generally has such a small value. When the desired value of condenser 38 is determined for any particular receiver, condensers 38 for all receivers similar thereto may be fixed condensers of the value ascertained.

Referring now to Fig. 2, the elements therein riably tuned, double-tuned coupling system I3',

`the -oscillator-modulator I4', the intermediatefrequency amplifier I6, the detector and automatic amplificationl control source I1, the audiolator-modulator I4', respectively. These switches selectively, connect a plurality of inductance coils (not shown) in circuit with their respective tuning condensers, Switches 40 and 4I are preferably ganged for unicontrol, as indicated by the dotted lines 42, with which are also ganged the switches 43, 44 and 45, as will be described hereinafter.

Each of the two tunable circuits 24' and 25' of coupling system I3' is selectively tunable over i the several frequency bands over which the selector and amplifier Il is tunable. This band selection is effected by switches 43 and 44, which selectively connect in circuit with the tuning condensers 3 I and 35, respectively, the inductance elements 30a and 34a, 30h and 34h, 38e and 34e, and 30d and 34d, for the several frevquency bands a, b, c, and d, respectively, which,

in the order given, are progressively higher in the frequency spectrum. Each inductance element may be effectively shunted by a small adjustably fixed trimmer condenser, as illustrated.

The first transfer means 26 of coupling system I3 comprises the series condensers 32 and 36, and the interconnecting condensers 38a, 38h, 38C and 38d, which are selectively connected into the coupling system by the switch,45 for the several bands a b, c, and d, respectively.

The second transfer means 21 of coupling system I3 comprises the condensers 21a, 2lb, 21e and 21d interconnecting the high potential ends of the inductance elements 38 and 34 for the several bands a b, c, and d, respectively. Preferably, the several inductance elements 30 are wound on one form and the several inductance elements 34 are wound on another form, with each form and its windings mounted within a separate grounded shield can, as illustrated.

The receiver of Fig. 2, including the coupling system I3, operates in each band in the same manner as that described for the single band receiver of Fig. l, so further discussion of such operation is believed unnecessary. From band to band, however, the effectiveness of the coupling system i3 varies, this variation being attained by altering the effectiveness of each transfer means as the switches are shifted for reception in the several bands. inasmuch as the present types of multi-band receivers have an attainable over-all gain-which becomes appreciably smaller as the bands are higher in the frequency spectrum, andy inasmuch as itis generally desirable to have an over-all gain which is more nearlyuniform from band to band, it is preferred to arrange the coupling system I3 so that it compensates for part, if not all, of such band-to-band variation in the over-all gain. This is conveniently accomplished by increasing the effectiveness of each of the transfer means 26 and 21 as the bands over which the, receiver is adjusted to operate are higher in the frequency 4the selector and amplifier II and for the oscilf spectrum. As'a result, the capacitances of the interconnecting condensers 38a, 38b3.8c and 38d for the first transfer means 26' and of thel interconnecting condensers 21a, 2lb, 21e and 21d for the second transfer means 21', increase in magnitude in the order named. y

Fig. 3 illustrates a multi-band coupling system ||3 which may be u sed in lieu of the multiband coupling system I3 of Fig. 2. For convenience, the elements illustrated in Fig. 3 corresponding to similar elements illustrated in Fig. 2 are designated by the same reference numerals, except that numbers in the 100 series are employed. The coupling system ||3 is a doubletuned system for each of the bands a, b and c, but is a single-tuned system for the highest frequency band d. The attendant sacrice of selectivity in band d is desirable in certain cases in order to obtain in such band as high a `transfer ratio as possible.

For each of the bands a, b, and c, the coupling system I I3 is electrically equivalent to that shown in Fig. 2. For simplicity, however, no separate condenser is provided as the interconnecting condenser of the second transfer means |21 in band a, recourse being had to the small inherent capacitance, represented in dotted lines as the capacitance |55, between the two tunable circuits |24 and |25, this capacitance existing primarily between the leads to the switches |43 and |44. The interconnecting condenser of the second transfer means |21 is, in band b, a turn |56 of an insulated wire |51 about the lead to the high potential terminal of inductance element |3017 and a turn |58 of the same insulated wire about the lead to the hi'gh potential terminal of inductance element I 34h, while, in band c, it is a turn |63 oian insulated Wire I 6| about the lead to the high potential terminal of inductance element |340, the other end of wire I6| being connected to the high potential terminal of inductance element |300. Also, for simplicity, no separate interconnecting condenser for the rst transfer means, |26 is provided for bands b and c, the condenser I38c being made to serve for both bands b and c.

To minimize any influence upon the coupling system of spurious resonances in any unused inf ductance element, switches and |52 are arranged to short-circuit the several inductance elements |30 and |34, respectively, for those bands below that to which the receiver is tuned. To avoid tuning directly, as in Fig. 2, the plate circuit of the preceding amplifier tube of the system, the tunable circuit |24 is coupled, for each of bands a, b, and c, inductively and capacitively to the untuned primary windings |53a, l53b and |530 by means of inductance elements I30a, |301) and |300, respectively, the windings I53a, I53b and I53e being selectively connected to the plate circuit of the preceding tube by the switch |50. The primary windings are preferably so poled that the inherent capacitive coupling between the respective primary windings and their corresponding inductance elements opposes, for bands a and b, and aids, for band c, the inductive coupling therebetween.

For band d, in which the coupling system I| operates as a single tunedsystem, the second tunable circuit |25 for vsuch band-is coupled inductively and capacitively to the input of the system by the primary. winding |53d which, as with the other primary windings 53, is controlled by the switch |50.

In view of the preceding descriptions relative Frequency range Band a, kilocycles 530-1525 Band b, megacycles M14-4.05 Band c, megacycles LO-10.0 Band d, megacycles 9.021.0

Inductance elements 30a and 34a-1391A turns-No. 32E close wound.

Inductance elements 3Ub and' 34h-34%; turns-No. 32E-88 turns per inch.

Inductance elements 30e and 34e-11% turns- No. 24E-36 turns per inch.

Inductance elements 30d and 34d-45/6 turns- No. 18E-16 turns per inch.

All coils wound on 1" diameter forms.

Mif- Condensers 3|' and 35 20-450 Condensers 32' and 36' 2300 Condenser 38a 40 Condenser 38h 150 Condenser 38o 250 Condenser 38d 500 Condenser 21a 1/2 Condenser 2lb Condenser 2'|c Estlmated Y Condenser 21d 1.5

For a receiver of the type shown in Fig. 2 with a coupling system of the type shown in Fig. 3 having the same frequency ranges setforth above and with its tuning condensers |3I and |35, its series condensers |32 and |36, its interconnecting condenser |38c, and its inductance elements I3IJa, |34a, |3019, |34b, |30c, |34c and |34d having the same values .as have the corresponding equipment set forth above, the following are the values of other pertinent equipment:

Inductance |53a 50 turns-v-No. 38 DSC close, wound over |30a.

Inductance I53b 15 turns-No. 38 DSC close, wound over |3I|b.

Inductance |,53c 7 turns-No. 3 2 DSC close, wound over |30c.

Inductance I53d 4 turns-No. 30 DSC interwound with |34d.

Condenser vI 38a 75 Inherent capacitance-| 55`::::: 1A Capacitance formed by wire |51 Estimated 1/2 Capacitance formed by Wire |6I 1 While I have described what I at present consider thepreferredembodiments of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from my invention, and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What is claimed is:

1. A coupling system` operative over a bandol' frequencies comprising; a Pair o1' tuned circuits having substantially similar reactance characteristics; and means for coupling said circuits to obtain a transfer characteristic over said band which is readily subject to control without disturbing the syntony of said circuits -including a self-reactance network all the reactances of which are of the same type and normally fixed in value, said network having two similar arms of equal effective reactive value individually and symmetrically included in circuit with said tuned circuits and having another arm symmetrically interconnecting said two similar arms, control of the transfer action of said system being effected by altering the effective reactive value of said interconnecting arm.

2. A coupling system operative over a band of frequencies including: two tuned circuits having substantially similar reactance characteristics; and means for coupling said circuits, said means having a transfer characteristic variable with frequency and comprising fixed `reactance elements of the same type and of substantially.

equal Value individually and symmetrically connected in circuit with said tuned circuits, and normally fixed reactance means effectively external to said circuits, and also of said type, coupling said individual reactance elements.

3. A coupling system operative over` a band of frequencies including: two tuned circuits having substantially similar reactance characteristics; and means for coupling said circuits, said means having a transfer characteristic variable with frequency and comprising a plurality of fixed reactance elements of the same type including at least one pair, the elements of each pair having substantially equal values and being individually and symmetrically connected in circuit with said tuned circuits, with at least one of said individual reactance elements of a first of said tuned circuits together with at least one other reactance element effectively external to said circuits forming a voltage dividing arrangement across the corresponding individual reactance element of the other of said tuned circuits. l

4. A coupling system operative over a band of frequencies including: a pair of l tuned circuits having similar reactance characteristics; and means for couplingsaid circuits, said means having a transfer characteristic variable with frequency Aand comprising fixed self-reactance elements of the same type and of substantially the same value individually and symmetrically connected in circuit with said tuned cicuits, and an. additional self-reactance element of said type connected effectively external to said circuits, said additional self-reactance element being adjustably fixed andsimilarly associated with each of said circuits so that adjustment of said one additional element, exclusively, varies the transfer actionv of said coupling means Without disturbing the syntony of said circuits.

5. A coupling system operative over a band of frequencies including: two tuned circuits having substantially similar reactance characteristics; and means for coupling said circuits, said means having a transfer characteristic variable with frequency and comprising at least one pair of fixed reactance elements of the same type, the reactance elements of each pair having substantially equal values and being individually and symmetrically connected in circuit with said tuned circuits with at least one pair of said reactance elements having a common terminal, and at least one additional fixed reactance element of the same type as said first reactance elements effectively external to said circuits, coupling corresponding points of a pair of said first reactance elements.

6. A coupling system operative over a band of frequencies including: two tuned circuits having substantially similar reactance characteristics; and means for coupling said circuits, said means having a transfer characteristic variable with frequency and comprising three fixed self-reactance elements of the same type, two thereof being of substantially equal value and symmetrically and individually connected in circuit with said tuned circuits and the third thereof being effectively external to said circuits and having a reactance substantially greater than, and interconnecting corresponding points of, said first two reactance elements.

\7. 'A coupling system operative over a band of frequencies including: two tuned circuits having substantially similar reactance characteristics; and means for coupling said circuits, said means having a transfer characteristic variable with frequency and comprising three' fixed reactance elements of the same type, two thereof being of substantially equal value and symmetrically and individually connected in portions of relatively low alternating potential of said circuits and having a common terminal, and the third thereof being effectively external to said circuits and interconnecting the other terminals of 'said first two reactance elements. I

8. A coupling system tunable over a band of frequencies comprising; a pair of tuned circuits having substantially similar reactance characteristics; and means for coupling said circuits to obtain a transfer characteristic over said band which is readily subject to control' without disturbing the syntony of said circuits including a capacitance network all the condensers of which are normally fixed, said network having two similar arms of equal effective capacitance individually and symmetrically included in circuit with said tuned circuits and having another arm effectively external to said circuits and symmetrically interconnecting said two similar arms, control of the transfer action of said system being effected by altering the effective capacitance of said interconnecting arm.

9. A coupling system operative over a band of frequencies including: two tuned circuits having substantially similar reactance characteristics; and means for coupling said circuits, said means having a transfer characteristic variable with frequency and comprising three normally fixed condensers, two thereof 'being of substantially equal value and symmetrically and individually connected in circuit with said tuned circuits and the third thereof effectively external t0 said circuits and interconnecting corresponding terminals of said first two condensers.

10. A coupling system operative over a band of frequencies including: two tuned circuits having substantially similar reactance characteristics; and two transfer means between said tuned circuits having transfer characteristics which vary with frequency in opposite senses, one of said transfer means comprising fixed reactance elecuits, and also of said type, coupling said individual reactance elements.

11. A coupling system operative over a band of frequencies including: a pair of inductively unrelated tuned circuits having substantially similar reactance characteristics; and two transfer means for coupling said circuits to obtain a desired transfer characteristic over said band, a nrst transfer means comprising self-reactance means coupling portions of said circuits of relatively high alternating potential and a second transfer means including self-reactance means effectively external to both of said circuits coupling portions thereof of relatively low alternating potential.- said two transfer means having transfer characteristics which vary oppositely with frequency. Y

12. A coupling system operative over a band of frequencies including: a pair of inductively unrelated tuned circuits having substantially similar reactance characteristics; and two transfer means for coupling said circuits to obtain a desired transfer characteristic over said band, a first transfer means comprising'ilxed self-'reactance means coupling portions of said circuits of relatively Vhigh alternating potential and a second transfer means comprising fixed self-reactance means of the same type as said first selfreactance means, at least a portion of which is effectively external to both of said circuits, coupling portions of said circuits of relatively low alternating potential, said two .transfer means having-transfer characteristics which vary oppositely with frequency.

13; A coupling system operative over a band of frequencies including? two tuned circuits havving substantially similar reactance characteristics; means for coupling said circuitsto obtain a desired transfer action over said band comprising a` first transfer means4 coupling portions of said circuits of relatively low alternating potential kand a second transfer means coupling portions of said circuits of relatively high alternating potential. each of said transfer means having aftransfer characteristic variable with frequency and one of said transfer means comprising `a plurality of fixed reactance elementsof the same type and of substantially equal value-individually and l symmetrically connected in circuit with said tuned circuits, and additional fixed reactance means, also of said type, coupling said individual reactance elements.

' 14. A coupling system operative over a band of'` frequencies including: two tuned circuits having 1 substantially similar reactance characteristics;

and means for4 coupling said circuits comprising `two transfer means between said circuits having transfer characteristics which vary in opposite senses with frequency, the transfer means having the transfer characteristic which. rises with frequency coupling portions `of said circuits of relatively' 'high alternating potential and the transfer means having the transfer characteristic which falls with frequency coupling portions of said circuits of relatively low alternating potential, the latter transfer means comprising fixed reactance elements of the same type and of substantially equal value symmetrically and individually connected in circuit with said tuned circuits, andA normallyxed reactance means, also of said type, coupling said individual reactance elements.l i

15. A coupling system operative over a band of frequencies including: two inductively unrelated tuned circuits having substantially similar reactance characteristics; and means for coupling said circuits comprising two transfer means between said circuits having transfer characteristics a,osc,sao

. individually connected in series in portions of said circuits of relatively low alternating potential and having a common terminal, and a normally fixed condenser of predetermined value less i than said two large fixed condensers interconnecting the other terminals thereof.

16. A coupling system tunable over a band of frequencies including: two tunable circuits having substantially similar reactance characteristics and conjointly operated tuning elements; and means for coupling said circuits, said means having a transfer characteristic variable with the frequency to which said circuits are tuned and comprising fixed reactance elements of the same type and of substantially equal value in- `dividually and symmetrically connected in circuit with said tunable circuits, and fixed reactance means, also of said type, coupling said individual reactance elements.

17. A couplingsystem tunable over a band of e frequencies including: a `pair4 of tunable circuits having substantially similar reactance characteristics and conjointly operated tuning elements; and two transfer means for coupling said circuits to obtainV a desired transfer characteristic over said band, a rst transfer means comprising fixed self-reactance means coupling por-l tions of said circuits of relatively high altemating potential and a second transfer means including fixed self-reactance means external to both of said circuits coupling portions thereof of relatively low-alternating potential, said two transfer means having transfer characteristics which vary oppositely with the frequency to which said circuits are tuned.

18. In a superheterodyne receiver. for operation over a band of vfrequencies and having an oscillator tunable by a tuning means: a radiofrequency coupling system tunable with said oscillator comprising; two tunable,circuits having substantially similar reactance characteristics,

yeach of which circuits includes a variable'tuning means, both said variable tuning means being similar to, and actuated coniointly with, the tunin'g means of said oscillator; and means for coupling said lcircuits so that the alignment thereof with respect.to each other and with respect to said oscillator is substantially unaffected by adjustment of the coupling of said coupling means and for control1ing`the frequency transfer characteristicA of said coupling means, said means comprising a self-reactance network all the reactances of which are of the same type and normally fixed in value, said network having two similar arms of equal veilfective reactive value individually and symmetrically included in circuit with said tuned circuits and having another arm symmetrically interconnecting said two similar arms, control of the transfer action of said system being effected by altering the effective reactive value of said interconnecting arm.

19. In a superheterodyne .receiver for operation over a band of frequencies and havingA an oscillator tunable by a tuning means: aradiofrequency coupling system tunable with said oscillator comprising; two tunable circuits having substantially similar reactance characteristics, each of which circuits includes a variable tuning means, and fixed self-reactance series aligning means, both said variable tuning means being similar to, and actuated conjolntly with, the tuning means of said oscillator; and means for coupling said circuits so that the alignment thereof with respect to each other and with respect to said oscillator is substantially unaffected by adjustment of the coupling of said coupling means and for controlling the frequency transfer characteristic of said coupling means, said means comprising said two aligning means and a reactance means of the same type as said two series aligning means and coupling corresponding electrically spaced points thereof.

20. In a superheterodyne receiver for operation over a band of frequencies and having an oscillator tunable by a tuning element thereof: a radio-frequency coupling system tunable with said oscillator comprising; two tunable circuits having substantially similar reactance 'characteristics, each of said circuits including a variable tuning element similar to and actuated conjointly with said oscillator tuning element; and means for coupling said circuits so that the alignment of said circuits with respect to each other and with respect to said oscillator is substantially unaffected by adjustment'of the coupling of said means, consisting of two transfer means having transfer characteristics which vary in opposite senses with the frequency `to which said circuits are tuned, said transfer means respectively coupling portions of said circuits of relatively low and lrelatively high alternating potential, and one of said transfer means including effectively fixed reactance elements of thesame type and of substantially equal value individually and symmetrically includedin circuit with said tunable circuits, and effectively fixed reactance means, also of said type, connecting corresponding electrically spaced points of said reactance elements.

21. A coupling system operative over two or more bands of frequencies including: a Apair of tuned circuits for each of said'bands, the two circuits of each of said pairs having similarl re- -actance characteristics; means for selectively connecting the two circuits of each of said pairs in circuit with the input and output of said coupling system, respectively; and means for cou- `pling the two circuits of each lof said pairs of circuits, said last-named means having for each of said bands a transfer characteristic variable with frequency and comprising for each of said bands fixed reactance elements of the same type and of substantially equal value individually and symmetrically connected in circuit with the corresponding tuned circuits, and a normally fixed reactance means, also of said type, coupling said individual reactance elements.

22. A coupling system operative over two or more bands of frequencies including: a pair of tuned circuits for each of said bands, the two circuits of each of said pairs having similar reactance characteristics; means for selectively connecting the two circuits of each of said pairs in circuit with the input and output of said coupling system, respectively; and means for coupling the two circuits of each of said pairs of circuits, said last-named means having for each of said bands a transfer characteristic variable with frequency and comprising for all of said bands' fixed reactance elements of the same type and of substantially equal value individually and symmetrically connected in circuit with `al1 th corresponding tuned circuits, a plurality of normally fixed reactance means of said type, one for each of said bands, having different reactive` values, and means forselectively connecting said normally fixed reactance means in circuit to couple said individual reactance elements.

v23. A coupling system operative over two or more bands of frequencies including: a pair of tuned circuits for each of said bands, the two circuits of each of said pairs having similar reactance characteristics; means for selectively connecting the two circuits of each of said pairs in circuit with the input and the output of said coupling'system, respectively; and two transfer means between the two circuits of each of said pairs, said two transfer means having transfer characteristics which vary in opposite senses with frequency, the transfer means for the severalpairs being proportioned to have a resultant transfer ratio which increases from pair to pair as the 'bands of frequencies to which the pairs are tuned are higher in the frequency spectrum.

`24. A coupling system tunable over two or more bands of frequencies including: a pair of tunable circuits for each of said bands, the rtwo circuits of each of said pairs having similar reactance characteristics and conjointly operated tuning elements; means for selectively connecting the two circuits of each of said pairs in circircuit with the input and output of said coupling system, respectively; and means for coupling the two circuits of each of said pairs of circuits, said last-named means having for each of said bands a transfer characteristic variable with the frequency to which said circuits are tuned and comprising for each of said bands fixed reactance elements of the same type and of substantially equal value individually and symmetrically connected in circuit 'with the corresponding' tunable circuits and a normally fixed the respective fixed reactance elements individual to the several pairs and for selectively connecting the two circuits of each pair with the input and the output vof said coupling system, respectively; and means for coupling the two circuits of each of said pairs to.obtain a substantially uniform transfer ratio over each of the tunable ranges of the several pairs and a transfer ratio which increases, from band to band, as the bands are higher in the frequency spectrum, comprising two transfer means between the two circuits of each of said pairs of circuits, said two transfer means for each pair having transfer characteristics which vary in opposite senses with variation in the frequency to which the associated pair of circuits aretuned, and coupling, respectively, the low and high potential portions of the associated circuits.

26. A coupling system tunable over two or more bands of frequencies including: a pair of tunable circuits for each of said bands, thev two circuits of each of said pairs having substantially similar reactance characteristics, and including iixed inductance elements individual to the sevlow potential ends of the inductance elements of spectrum, comprising;

each of the two groups of corresponding circuits of said pairs, said circuits including a variable tuning condenser for each of said groups, and means for selectively connecting said tuning condensers in circuit with the respective xed inductance elements individual to the several pairs and' for selectively connecting the two circuits of each pair with the input and the output of said coupling system, respectively; and means for coupling the two circuits of each of said pairs to obtain a substantially uniform transfer ratio over each of the tunable ranges of the several pairs and, a transfer ratio which increases from band to band as the bands are higher in the frequency a first transfer means be tween the two circuits of each of said pairs of circuits having a transfer characteristic which varies inversely with frequency and comprising relatively large fixed condensersof substantially the same value connected respectively between said common low potential ends of each of said groups of circuits and ground, a plurality of normally fixed condensers, and means for selectively connecting said normally fixed condensers between said common low potential ends; and a second transfer means between the two circuits of each of said pairs of circuits having a transfer characteristic which varies directly with frequency and comprising relatively small normally fixed condensers connected between the high potential ends of the inductance elements for each of said pairs of circuits.

NELSON P. CASE. 

